A preliminary single-channel, high-speed, stopped-flow calorimeter has been constructed to evaluate mixing speeds, flow velocities, pressure drops, and thermal detection sensitivity. The system uses a variable-speed flywheel and an electrically actuated clutch to transfer sufficient energy to the two drive syringes to mix reagent volumes of 200 microliters in 5 milliseconds or less. The inlet tubes from the drive syringes, as well as the mixer and the detection chamber, are kept at constant temperature between 25 and 50 deg C. An ultra-fast thermistor (1 ms) is located in the detection chamber to measure the temperature rise during the reaction. A new drive cam is used, which produces a constant acceleration of the syringes. The mixing chamber contains a quartz observation tube to allow optical measurement of the reaction. The observation tube 'floats' in O-ring seals to reduce shock loads during mixing. The latest change to the instrument involved adding a second, identical channel to allow differential measurements to be made.